1. Field of the Invention
This invention relates generally to electronic utility metering devices and, more specifically, to an apparatus and method to detect tampering with an electronic utility meter device having an internal disconnect switch.
2. Description of the Prior Art
Electrical metering of residential energy has historically been accomplished using electromechanical meters that sense electromagnetic fields in proximity to voltage coils and current carrying conductors. These electromechanical meters are generally effective in metering electrical energy consumption, but are incapable of providing, in a cost effective manner, numerous additional functions currently being demanded by the utility industry. For example, modem utility companies desire metering devices capable of metering not only total energy consumption, but also many other electrical parameters (e.g., time-of-use).
The industry has responded to this demand by developing completely electronic metering devices. Electronic meters cost effectively provide the utility with measurements of a number of electrical parameters, as well many other advantages. For example, to reduce the costs associated with reading meters, utility companies use electronic metering devices with built in communications modules capable of communicating meter reading data back to the utility's master station. Such communications networks obviate the need for human meter readers to go to individual subscriber locations to read meters.
Another function the utility industry demands of electronic metering devices is the capability to connect or disconnect service to individual subscriber locations. For a variety of reasons, utilities are routinely and frequently required to connect and disconnect service. For example, a utility may need to disconnect service when a subscriber moves out of a particular location and then later reconnect service to the same location when another subscriber moves in. Likewise, utility companies are sometimes forced to disconnect service to a subscriber who does not pay, and then reconnect service when the subscriber's account is settled. A number of electronic metering devices currently exist that provide connect/disconnect capability through the use of disconnect switches internal to the meter.
However, the utilities are typically forced to send a human to the subscriber location to connect or disconnect service by closing or opening the disconnect switch at the meter itself. This approach is particularly time-consuming (and, thus, costly) on college campuses, military bases, apartment complexes, and the like, where the rate of subscriber turnover is high.
The cost of connecting/disconnecting service is greatly reduced if the utility operates the internal disconnect switch from a remote location, such as the utility's master station. In this way, service could be routinely connected and disconnected to subscriber locations, without the utility expending the cost associated with sending a human to the site.
Unfortunately, many unscrupulous subscribers whose service has been disconnected have attempted to restore service themselves, without compensating the utility for the benefit, by tampering with the meter. One common tampering technique is to place a resistor in parallel with the meter, thus causing current to circumvent the meter (and the disconnect switch), essentially reconnecting the circuit without actually closing the disconnect switch.
FIG. 1 illustrates a typical electrical energy meter 10 with an internal disconnect switch 40. The meter 10 comprises a source side current sensor 30, a source side voltage sensor 32, a disconnect switch 40 containing two electromechanical solenoid driven switches 42 and 44, and a microprocessor 50.
Electrical energy (L1 in; L2 in) is supplied by a voltage source 12 and delivered, via source side feeder lines 20A and 20B, through meter 10, to an electrical load at a subscriber location 14. Disconnect switch 40 is interposed onto the feeder lines, separating source side feeder lines 20A and 20B from load side feeder lines 20C and 20D. The electrical energy delivered to the load on load side feeder lines 20C and 20D is given by L1 out and L2 out, respectively.
When disconnect switch 40 is closed, electrical energy (L1 out; L2 out) should be delivered to subscriber location 14, and when disconnect switch 40 is open, no electrical energy should be delivered to subscriber location 14.
In the situation in which a tamperer has installed lines 26A and 26B, current is delivered to the load at subscriber location 14, thereby circumventing meter 10. Although disconnect switch 40 is open, current will still be delivered to the subscriber location 14, but current sensor 30 will not detect the delivered current. Similarly, a line voltage will occur on the load side feeder lines 20C and 20D. However, since a source side voltage is always present, source side voltage sensor 32 is unable to detect the tampering. Thus, the subscriber will be supplied with electrical energy without the utility having the ability to meter the usage.
Similarly, if disconnect switch 40 is closed (i.e., the utility has not disconnected service to subscriber location 14), the tamperer might still provide himself with free electrical energy. In this case, lines 26A and 26B might be equipped with resistance such that some, but not all, of the delivered current would be delivered over lines 26A and 26B, circumventing current sensor 30. Thus, the utility would meter only a portion of the actual energy being delivered.
The industry has responded to the tampering problem by installing various tamper detection mechanisms into existing meters, or by developing meters with built-in tamper detection mechanisms. Many of these approaches attempt to detect tampering by measuring the current being drawn by the subscriber location (i.e., the load-side current), comparing the measured current to a predetermined expected value, and concluding whether or not less current is being drawn than expected. If it is concluded that too little current is being drawn, it is presumed that a tamper condition exists. This approach is highly subjective since one has to essentially guess at the expected current value. Further, since there are numerous reasons why a subscriber legitimately might be using less-than-usual current for a period of time, this approach tends to generate a large number of false alarms. Moreover, tamper detection mechanisms currently used in the art are expensive to implement, especially in residential environments where the per unit cost is multiplied by tens of thousands, or even millions of meters.
Thus, there is a need for a very low cost tamper detection mechanism for electronic meters with internal disconnect switches that uses an objective approach to determine whether a tamper condition exists at the meter.